1. Brief Description Of The Prior Art
Mounting concerns for all forms of environmental pollution have resulted in governmental requirements that liquids stored in tanks be safeguarded against undesirable leakage to the surroundings. It is widely required by U.S. federal and state legislation that double containment systems be provided for almost all liquids which are incompatible with ground water, or damaging to vegetation, and particularly in the case of underground liquid storage tanks. The latter category includes thousands of previously buried gasoline and diesel storage tanks which have been in the ground for many years, and are often in an advanced stage of deterioration. These tanks must either be replaced with new, double-walled tanks, or converted to an effective double-containment system. The former alternative often poses prohibitive expense, with the result that, unless an effective in-situ conversion can be accomplished, the small business depending upon the underground liquid storage tank, such as an automobile service station, frequently cannot continue to operate.
As indicated, the required environmental safeguards against environmental pollution from subterranean liquid storage have generally required the use of some form of double-walled tank or container--a form of redundancy which alone, of course, would theoretically merely require twice as long for the undesirable pollution to occur. In recognition of this, widespread efforts have gone forward, and in some cases have been required by certain regulations, to provide a system for monitoring the integrity of the containment system, and for testing the two walls for leakage or failure. In this way, at worst, a forewarning of imminent environmental pollution is had, and at best, measures may be taken to repair leaks in the system, or otherwise restore its liquid containment integrity.
One type of containment system which has heretofore been proposed is the double-walled containment system described in Hendershot et al U.S. Pat. No. 4,796,676 and Hendershot et al pending U.S. patent application Ser. No. 429,073, assigned to World Enviro Systems, Inc. These patents advocate the placement inside of a steel or Fiberglas.RTM. outer tank, which in most cases will be an existing single wall tank, of a flexible, internal liner or tank which actually contains the liquid to be stored in the double-walled tank system. This flexible liner or internal tank is configured to fit closely against the internal wall of the external tank, and may be forced into this position of lining the external tank by the use of internal air pressure and/or by use of an evacuation of the interstices existent between the external wall of the internal tank or liner, and the internal wall of the external tank.
The described system, using a rigid external tank of Fiberglas.RTM. or steel, and an internal flexible tank or liner, affords the advantage that, by the use of the flexible internal tank or liner, it is possible to install such an internal tank by collapsing the flexible material and forcing it into the external tank through an existing manway, and then inflating or drawing the internal tank into its operative position in juxtaposition to the inner side of the external tank. Similarly, if it should be known that the internal liner has developed any type of leak over an extended period of usage, it is possible to extricate this flexible internal tank in the same way that it was originally installed, i.e. by removing it through a manway or other existing opening in the external tank, and then repairing it outside of the external tank, or completely replacing it with a new liner, all without the expense of excavating the entire system from the earth in order to make the necessary repairs.
As a part of the system described in U.S. Pat. No. 4,796,676 and U.S. patent application Ser. No. 429,073, the means by which the interstices between the flexible internal tank and the rigid external tank is evacuated is illustrated and described in detail. As a part of this system for evacuating this space, the system also includes means by which the negative air pressure within this space is continuously monitored, and can be restored periodically, as may be necessary to account for inherent and unavoidable slow leakage due to gas permeability of both the flexible internal tank and the rigid external tank. The system which is used to monitor and to evacuate the interstices between the flexible internal tank and the rigid external tank also is capable of removing liquid from a sump or low zone between the tanks when such liquid accumulates at that location.